This invention relates generally to phosphate conversion coating for metals and more specifically to a process and material which forms conversion coatings having a reduced crystal size and coating weight by including certain phosphates and phosphonates which contain free alcoholic hydroxyl groups.
Phosphate conversion coatings on metals (i.e., steel and iron, zinc, galvanized steel, cadmium, and aluminum) are used for a variety of reasons. They are indispensible as adhesion promoters and they will improve the corrosion resistance for metal goods that have to be painted. They can also be used as a carrier base for a rust preventive oil, and they are used as lubricant carriers for metal cold forming operations and in lubricated bearings and other lubricated friction surfaces. Phosphate coatings are formed by contacting the metal surface with an acidic phosphate solution. The acid will dissolve some of the base metal and at the same time deposit an insoluble phosphate onto the surface. This is caused by the fact that the dissolution of the metal decreases the acidity near the surface area. To accelerate the speed of coating, the phosphate coating solutions (applied by immersion, spray, or roll-on) are mostly used at elevated temperatures and accelerators in the form of oxidizing compounds are added.
There are two basic types of phosphate solutions. The first one uses the dissolved base metal itself to form the phosphate coatings. It is essentially a dilute phosphoric acid solution with the acidity reduced to a somewhat lower level with an alkali and which contains an accelerator. These types of products are useful exclusively as a paint base, mainly for steel, and they are called iron phosphate coatings in the art. The coatings are flexible so that coil stock can be pre-painted and then formed without the paint cracking. However, painted goods using an iron phosphate base have less corrosion resistance than those having phosphate coatings of other types and therefore are not used in an outdoor environment or in other heavy duty applications.
The other type contains divalent metal salts that will form insoluble phosphates on a metal surface. The products most widely used contain acid zinc and zinc-nickel phosphates, but products using manganese, zinc-calcium and zinc-magnesium are also on the market. Of those six groups, the zinc and zinc-nickel phosphate compounds are the easiest to operate. They are used in all the afore-mentioned types of applications and are superior in corrosion resistance to iron phosphate under paint. Manganese and zinc-manganese phosphates are used as lubricant carriers in sliding friction service because of the superior hardness of these deposits. Zinc-magnesium phosphates do not have any advantage over zinc phosphates and are not widely used. Zinc phosphate, zinc-nickel phosphate, manganese phosphate, and zinc-manganese phosphate coatings are all of a more or less coarse crystalline structure. While this might be advantageous for some lubrication applications, where it is desirable to absorb a maximum of the lubricant on the surface, it is detrimental in most other applications, especially in under-paint service. Here it leads to a higher use of paint, the painted surface will be less glossy unless the paint thickness is increased above that necessary for an iron phosphate pretreatment, and especially important is that the metal cannot be bent anymore after painting because such bending or other deformation will result in the loss of paint adhesion. For this reason, only iron phosphate coatings can be used on prepainted coil stock, although zinc or zinc-nickel phosphate would result in a longer service life of the painted goods. The draw-backs of the coarse crystalline structure of phosphates other than iron phosphates for many applications have been recognized over the years and several methods have been used to overcome these problems.
One way to obtain a finer, denser crystal size uses a pretreatment prior to phosphate coating. Generally, metal parts to be phosphated with a crystal forming product have to be thoroughly cleaned beforehand. The most efficient way to do that is by using hot and strongly alkaline detergent solutions. A steel surface cleaned this way will result in especially coarse phosphate deposits. However, if the metal is rinsed with certain solutions before phosphating (mostly based on colloidal titanium phosphates), the deposits are finer and denser, although not fine enough to become flexible. Most phosphate coating lines for goods to be painted employ these pre-rinses (or, instead of an extra rinse, these compounds are added to the cleaning solution). These preconditionings of the metal surfaces are not sufficient to obtain micro-crystalline coatings.
The other approach has been to change the phosphate coating solution itself. One method is the use of a bath containing the above mentioned zinc-calcium phosphate. This method results in truly dense, micro-crystalline coatings. However, in spite of the good deposits obtained with zinc-calcium baths, they are not widely used, mainly because of inherent draw-backs. They are energy inefficient, as the baths have to be operated at relatively high temperatures. The baths form more scale on heating elements, tank walls, and piping than other baths, but mainly it is difficult to keep the baths in a good coating condition because of an inherent instability.
Another method to obtain micro-crystalline deposits is the addition of condensed phosphate salts, such as for example, sodium pyrophosphate, sodium tripolyphosphate, or sodium hexametaphosphate. A phosphate coating bath of this type is even harder to control than the zinc-calcium bath. Very small amounts (depending on temperature and concentration, 50-300 parts per million) of condensed phosphates are necessary to obtain micro-crystallinity. A small excess will stop the coating process completely. On the other hand, condensed phosphates are very instable in the acidic phosphate bath and under some conditions, might have a half life of only a few minutes, plus, they are used up rapidly in the coating itself. A line employing condensed phosphate additions would have to use microprocessor controls.
Another method that has been disclosed is the addition of glycerophosphoric acid and its salts. These chemicals result in a fairly good reduction of crystal size, although from my experience not as much as with the zinc-calcium phosphate products or zinc phosphate baths with condensed phosphate additions. The coating weigh reduction is only moderate. Such glycerophosphate baths are disclosed, for example, in British Pat. No. 876,250 and U.S. Pat. Nos. 3,109,757 and 3,681,148.
In my own experimentations, I needed between 0.8 and 1.5% by weight of the glycerophosphate compound in a phosphate coating bath. This approaches the concentration of the coating chemicals in the bath (i.e., zinc, phosphoric acid, and accelerators). The costs per weight unit of glycerophosphates are a magnitude higher than the ones of the coating chemicals. Also, straight chain aliphatic acid esters like glycerphosphoric acid are subject to de-esterification, which would make frequent replenishing necessary. Perhaps for these reasons, to my knowledge, such baths have had limited, if any, commercial use.
Sealing rinses which are applied after phosphating the metal are disclosed in U.S. Pat. No. 3,957,543 (an aqueous solution of technical grade phytic acid) and U.S. Pat. No. 4,220,485 (an aqueous solution of phosphoric acid; an acid soluble zinc compound; a heavy metal accelerator or a crystal refiner such as nickel or calcium nitrate, and a phosphonate corrosion inhibitor such as hydroxyethylidene-1,1-diphosphonic acid). In U.S. Pat. No. 3,900,370 anodized aluminum surfaces are sealed with a sealer including calcium ions and a water soluble phosphonic acid such as hydroxyethylidene-1, 1, -diphosphonic acid or its water soluble salt at temperatures of from 90.degree. C. to the solution boiling point.
I have now found that such phosphorus containing compounds prove to be effective in significantly reducing crystal size and coating weight when used directly in the phosphate conversion coating forming baths as crystal refiners. They also provide phosphating baths which are easily controlled, which do not result in excessive scale formation, which are stable, and which can be operated at lower temperatures than previously required. The resulting coatings provide an excellent flexible paint base with good corrosion resistance despite the reduced coating weight. These compounds belong to the class of acidic, organic phosphates and phosphonates. More specifically, they all possess at least one free alcoholic hydroxyl group in the molecule. The phosphates used in this invention are acid esters of cyclic or branched aliphatic polyols.